Catalyst article and the use thereof for filtering fine particles

ABSTRACT

The present invention provides catalyst article, and its use in an exhaust system for internal combustion engines, is disclosed. The catalyst article catalyst article comprises: a substrate which is a wall-flow filter having an inlet end and an outlet end and an axial length L therebetween, a plurality of inlet channels extending from the inlet end and a plurality of outlet channels extending from the outlet end, wherein the plurality of inlet channels comprise a first catalyst composition extending from the inlet or outlet end for at least 50% of L and the plurality of outlet channels comprise a second catalyst composition extending from the outlet or inlet end for at least 50% of L, wherein the first and second catalyst compositions overlap by at most 80% of L, and wherein the first and second catalyst compositions each independently comprise a particulate oxygen storage component (OSC) having a first D90 and a particulate inorganic oxide having a second D90 and: i) the first D90 is less than 1 micron and the second D90 is from 1 to 20 microns; or ii) the second D90 is less than 1 micron and the first D90 is from 1 to 20 microns.

FIELD OF THE INVENTION

The present disclosure relates to a catalyst article for treating exhaust gas emissions from gasoline engines, in particular to a wall-flow filter suitable for treating fine particulate emissions having a size of less than 23 nm.

BACKGROUND OF THE INVENTION

Positive ignition engines cause combustion of a hydrocarbon and air mixture using spark ignition. In contrast, compression ignition engines cause combustion of a hydrocarbon by injecting the hydrocarbon into compressed air. Positive ignition engines can be fuelled by gasoline fuel, gasoline fuel blended with oxygenates including methanol and/or ethanol, liquid petroleum gas or compressed natural gas. Positive ignition engines can be stoichiometrically operated engines or lean-burn operated engines.

A three-way catalyst (TWC) typically contains one or more platinum group metals, particularly those selected from the group consisting of platinum, palladium and rhodium. TWCs are intended to catalyse three reactions simultaneously:

-   -   (i) oxidation of carbon monoxide to carbon dioxide,     -   (ii) oxidation of unburned hydrocarbons to carbon dioxide and         water; and     -   (iii) reduction of nitrogen oxides to nitrogen and oxygen.

These three reactions occur most efficiently when the TWC receives exhaust gas from an engine running at or about the stoichiometric point. As is well known in the art, the quantity of carbon monoxide (CO), unburned hydrocarbons (HC) and nitrogen oxides (NO_(x)) emitted when gasoline fuel is combusted in a positive ignition (e.g. spark-ignited) internal combustion engine is influenced predominantly by the air-to-fuel ratio in the combustion cylinder. An exhaust gas having a stoichiometrically balanced composition is one in which the concentrations of oxidising gases (NO_(x) and O₂) and reducing gases (HC and CO) are substantially matched. The air-to-fuel ratio that produces this stoichiometrically balanced exhaust gas composition is typically given as 14.7:1.

Theoretically, it should be possible to achieve complete conversion of O₂, NO_(x), CO and HC in a stoichiometrically balanced exhaust gas composition to CO₂, H₂O and N₂ (and residual O₂) and this is the duty of the TWC. Ideally, therefore, the engine should be operated in such a way that the air-to-fuel ratio of the combustion mixture produces the stoichiometrically balanced exhaust gas composition.

A way of defining the compositional balance between oxidising gases and reducing gases of the exhaust gas is the lambda (λ) value of the exhaust gas, which can be defined according to equation (1) as: Actual engine air-to-fuel ratio/Stoichiometric engine air-to-fuel ratio,  (1) wherein a lambda value of 1 represents a stoichiometrically balanced (or stoichiometric) exhaust gas composition, wherein a lambda value of >1 represents an excess of O₂ and NO_(x) and the composition is described as “lean” and wherein a lambda value of <1 represents an excess of HC and CO and the composition is described as “rich”. It is also common in the art to refer to the air-to-fuel ratio at which the engine operates as “stoichiometric”, “lean” or “rich”, depending on the exhaust gas composition which the air-to-fuel ratio generates: hence stoichiometrically-operated gasoline engine or lean-burn gasoline engine.

It should be appreciated that the reduction of NO_(x) to N₂ using a TWC is less efficient when the exhaust gas composition is lean of stoichiometric. Equally, the TWC is less able to oxidise CO and HC when the exhaust gas composition is rich. The challenge, therefore, is to maintain the composition of the exhaust gas flowing into the TWC at as close to the stoichiometric composition as possible.

Of course, when the engine is in steady state it is relatively easy to ensure that the air-to-fuel ratio is stoichiometric. However, when the engine is used to propel a vehicle, the quantity of fuel required changes transiently depending upon the load demand placed on the engine by the driver. This makes controlling the air-to-fuel ratio so that a stoichiometric exhaust gas is generated for three-way conversion particularly difficult. In practice, the air-to-fuel ratio is controlled by an engine control unit, which receives information about the exhaust gas composition from an exhaust gas oxygen (EGO) (or lambda) sensor: a so-called closed loop feedback system. A feature of such a system is that the air-to-fuel ratio oscillates (or perturbates) between slightly rich of the stoichiometric (or control set) point and slightly lean, because there is a time lag associated with adjusting air-to-fuel ratio. This perturbation is characterised by the amplitude of the air-to-fuel ratio and the response frequency (Hz).

The active components in a typical TWC comprise one or both of platinum and palladium in combination with rhodium, or even palladium only (no rhodium), supported on a high surface area oxide, and an oxygen storage component.

When the exhaust gas composition is slightly rich of the set point, there is a need for a small amount of oxygen to consume the unreacted CO and HC, i.e. to make the reaction more stoichiometric. Conversely, when the exhaust gas goes slightly lean, the excess oxygen needs to be consumed. This was achieved by the development of the oxygen storage component that liberates or absorbs oxygen during the perturbations. The most commonly used oxygen storage component (OSC) in modern TWCs is cerium oxide (CeO₂) or a mixed oxide containing cerium, e.g. a Ce/Zr mixed oxide.

In addition to the carbon monoxide (CO), unburned hydrocarbons (HC) and nitrogen oxides (NO_(x)) emitted when gasoline fuel is combusted in a positive ignition, all of which can be treated with the TWC catalyst, there are particulate emissions which need to be considered.

Ambient PM is divided by most authors into the following categories based on their aerodynamic diameter (the aerodynamic diameter is defined as the diameter of a 1 g/cm³ density sphere of the same settling velocity in air as the measured particle):

-   -   (i) PM-10—particles of an aerodynamic diameter of less than 10         μm;     -   (ii) Fine particles of diameters below 2.5 μm (PM-2.5);     -   (iii) Ultrafine particles of diameters below 0.1 μm (or 100 nm);         and     -   (iv) Nanoparticles, characterised by diameters of less than 50         nm.

Since the mid-1990's, particle size distributions of particulates exhausted from internal combustion engines have received increasing attention due to possible adverse health effects of fine and ultrafine particles. Concentrations of PM-10 particulates in ambient air are regulated by law in the USA. A new, additional ambient air quality standard for PM-2.5 was introduced in the USA in 1997 as a result of health studies that indicated a strong correlation between human mortality and the concentration of fine particles below 2.5 μm.

Interest has now shifted towards nanoparticles generated by diesel and gasoline engines because they are understood to penetrate more deeply into human lungs than particulates of greater size and consequently they are believed to be more harmful than larger particles, extrapolated from the findings of studies into particulates in the 2.5-10.0 μm range.

Size distributions of diesel particulates have a well-established bimodal character that correspond to the particle nucleation and agglomeration mechanisms. Diesel PM is composed of numerous small particles holding very little mass. Nearly all diesel particulates have sizes of significantly less than 1 μm, i.e. they comprise a mixture of fine, i.e. falling under the 1997 US law, ultrafine and nanoparticles.

It is understood that diesel particulate filters, such as ceramic wallflow monoliths, may work through a combination of depth and surface filtration: a filtration cake develops at higher soot loads when the depth filtration capacity is saturated and a particulate layer starts covering the filtration surface. Depth filtration is characterized by somewhat lower filtration efficiency and lower pressure drop than the cake filtration.

PM generated by positive ignition engines has a significantly higher proportion of ultrafine, with negligible accumulation and coarse mode compared with that produced by diesel (compression ignition) engines, and this presents challenges to removing it from positive ignition engine exhaust gas in order to prevent its emission to atmosphere. In particular, since a majority of PM derived from a positive ignition engine is relatively small compared with the size distribution for diesel PM, it is not practically possible to use a filter substrate that promotes positive ignition PM surface-type cake filtration because the relatively low mean pore size of the filter substrate that would be required would produce impractically high backpressure in the system.

Furthermore, generally it is not possible to use a conventional wallflow filter, designed for trapping diesel PM, for promoting surface-type filtration of PM from a positive ignition engine in order to meet relevant emission standards because there is generally less PM in positive ignition exhaust gas, so formation of a soot cake is less likely; and positive ignition exhaust gas temperatures are generally higher, which can lead to faster removal of PM by oxidation, thus preventing increased PM removal by cake filtration. Depth filtration of positive ignition PM in a conventional diesel wallflow filter is also difficult because the PM is significantly smaller than the pore size of the filter medium. Hence, in normal operation, an uncoated conventional diesel wallflow filter will have a lower filtration efficiency when used with a positive ignition engine than a compression ignition engine.

Emission legislation in Europe from 1 Sep. 2014 (Euro 6) requires control of the number of particles emitted from both diesel and gasoline (positive ignition) passenger cars. For gasoline EU light duty vehicles the allowable limits are: 1000 mg/km carbon monoxide; 60 mg/km nitrogen oxides (NO_(x)); 100 mg/km total hydrocarbons (of which ≤68 mg/km are non-methane hydrocarbons); and 4.5 mg/km particulate matter ((PM) for direct injection engines only). A PM number standard limit of 6.0×10¹¹ per km has been set for Euro 6, although an Original Equipment Manufacturer may request a limit of 6×10¹² km⁻¹ until 2017. In a practical sense, the range of particulates that are legislated for are between 23 nm and 3 μm.

In the United States, on 22 Mar. 2012, the State of California Air Resources Board (CARB) adopted new Exhaust Standards from 2017 and subsequent model year “LEV III” passenger cars, light-duty trucks and medium-duty vehicles which include a 3 mg/mile emission limit, with a later introduction of 1 mg/mile possible, as long as various interim reviews deem it feasible.

The new Euro 6 emission standard presents a number of challenging design problems for meeting gasoline emission standards. In particular, how to design a filter, or an exhaust system including a filter, for reducing the number of PM gasoline (positive ignition) emissions, yet at the same time meeting the emission standards for non-PM pollutants such as one or more of oxides of nitrogen (NO_(x)), carbon monoxide (CO) and unburned hydrocarbons (HC), all at an acceptable back pressure, e.g. as measured by maximum on-cycle backpressure on the EU drive cycle.

There have been a number of recent efforts to combine TWCs with filters for meeting the Euro 6 emission standards.

WO2014125296 discloses a positive ignition engine comprising an exhaust system for a vehicular positive ignition internal combustion engine. The exhaust system comprises a filter for filtering particulate matter from exhaust gas emitted from the vehicular positive ignition internal combustion engine coated with a three-way catalyst washcoat. The washcoat comprises a platinum group metal and a plurality of solid particles, wherein the plurality of solid particles comprises at least one base metal oxide and at least one oxygen storage component which is a mixed oxide or composite oxide comprising cerium. The mixed oxide or composite oxide comprising cerium and/or the at least one base metal oxide has a median particle size (D50) less than 1 μm.

US20090193796 discloses an emission treatment system downstream of a gasoline direct injection engine for treatment of an exhaust gas comprising hydrocarbons, carbon monoxide, nitrogen oxides and particulates, the emission treatment system comprising a catalysed particulate trap comprising a three-way conversion (TWC) catalyst coated onto or within a particulate trap. In the description and Examples provided, a catalyst coating (also referred to as a layer or layered catalyst composite) is prepared from a slurry mixture of a solution of desired precious metal compounds and at least one support material, such as finely divided, high surface area, refractory metal oxide. The slurry mixture is comminuted, e.g. in a ball mill or other similar equipment, to result in substantially all of the solids having particle sizes of less than about 20 μm, i.e. between about 0.1-15 m in an average diameter [known as “D50”]. In the Examples, comminution by milling of alumina was done so that the particle size of 90% [known as “D90”] of the particles was 8-10 m. Comminution of a ceria-zirconia composite was done by milling to a D90 particle size of <5 μm.

In GB2514875 the inventors considered the use of washcoat compositions comprising milled cerium/zirconium mixed oxides for use in three-way catalysts for coating filters, such as those disclosed in US 20090193796, for low back-pressure applications. Here it was found that by milling cerium/zirconium mixed oxides, although the backpressure decreased with decreasing D50 of the cerium/zirconium mixed oxide, simultaneously, the three-way catalyst activity was significantly reduced, particularly for CO and NO_(x) emissions.

Accordingly, it is an object to provide an improved TWC catalyst filter tackling the disadvantages of the prior art, or at least to provide a commercially useful alternative thereto. More specifically, it is an object to provide a catalyst article that enables treatment of exhaust gas to address fine sub-23 nm particulate emissions.

Accordingly, in a first embodiment there is provided a catalyst article for treating an exhaust gas from a positive ignition internal combustion engine, the article comprising:

a substrate which is a wall-flow filter having an inlet end and an outlet end and an axial length L therebetween, a plurality of inlet channels extending from the inlet end and a plurality of outlet channels extending from the outlet end,

wherein the plurality of inlet channels comprise a first catalyst composition extending from the inlet or outlet end for at least 50% of L and the plurality of outlet channels comprise a second catalyst composition extending from the outlet or inlet end for at least 50% of L, wherein the first and second catalyst compositions overlap by at most 80% of L, and

wherein the first and second catalyst compositions each independently comprise a particulate oxygen storage component (OSC) having a first D90 and a particulate inorganic oxide having a second D90 and:

-   -   i) the first D90 is less than 1 micron and the second D90 is         from 1 to 20 microns; or     -   ii) the second D90 is less than 1 micron and the first D90 is         from 1 to 20 microns.

The present invention will now be further described. In the following passages different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

The present inventors found that the problem of reduced three-way catalyst activity observed in GB2514875 can be addressed by using a very fine source of cerium/zirconium, such as can be provided by a sol material instead of milling cerium/zirconium mixed oxides. They also found that back pressure can also be reduced to desirable levels by the careful selection of the particle sizes of the cerium/zirconium mixed oxide component and the inorganic oxide materials such as alumina.

While conducting research of further compositions to achieve lower backpressure of the coated filter and maintain an overall good filtration efficiency, the inventors have surprisingly discovered that a combination of a nano-solution of cerium/zirconium mixed oxide (OSC material) with a particle size D90 of <1 μm and a first inorganic oxide with a D90 from 1 μm to 20 μm or ‘vice versa’, has achieved excellent benefits in reducing the particulate number count at tailpipe in the sub 23 nm range (such as 10-23 nm).

The advantage of this discovery is clear in addressing the associated health risks with ultra fine soot particles mentioned above and well referenced and discussed recently in the International Journal of Environmental Research and Public Health (Int. J. Environ. Res. Public Health 2018, 15, 304). Indeed, the global acceptance of ultrafine particles and the impact on cardiorespiratory and central nervous system health issues has driven the tightening of global legislative limits for automotive applications. Beyond today's emissions limits, legislators look to further improve air quality and one area for consideration is particulate matter, currently measured as PN or Particulate Number for Euro 6d standards.

Currently the measurement requirement of PN is for particles of 23 nm or greater. One obvious aspect to consider tightening legislation as measurement accuracy improves, is to consider the particle sizes below 23 nm. The described invention therefore lends itself to solving the technical challenge of anticipated tightening of particulate emissions legislation beyond Euro6d and bringing about the desired health benefits of reducing ultra-fine particles into the atmosphere.

The present invention provides a catalyst article for treating an exhaust gas from a positive ignition internal combustion engine. A catalyst article as used herein refers to a component of an exhaust gas system, in particular a TWC catalyst for the treatment of an exhaust gas. Such catalytic articles provide a supported catalyst for the treatment of gases brought into contact with the catalyst. The catalyst article described herein comprises the multiple sub-components described herein.

The catalyst article may be close-coupled to the engine. By “close-coupled” it is meant that the catalyst article is for installation in close proximity to the exhaust manifold of an engine. That is, preferably the catalyst article is for installation in the engine bay and not on the underfloor of the vehicle. Preferably the catalyst article is the first catalyst article provided downstream of the engine manifold. The close-coupled position is very hot due to proximity to the engine.

The article comprises a substrate which is a wall-flow filter having an inlet end and an outlet end and an axial length L therebetween, a plurality of inlet channels extending from the inlet end and a plurality of outlet channels extending from the outlet end. Wall flow filters are well known in the art and typically comprise a ceramic porous filter substrate having a plurality of inlet channels and a plurality of outlet channels, wherein each inlet channel and each outlet channel is defined in part by a ceramic wall of porous structure, wherein each inlet channel is separated from an outlet channel by a ceramic wall of porous structure.

Typical lengths L are from 2-12 inches long (5.1-30.5 cm), preferably 3-6 inches (7.6-15.2 cm) long. Cross sections are preferably circular and may typically have 4.66 and 5.66 inch (11.8 cm and 14.4 cm) diameter filters. However, cross-section can also be dictated by space on a vehicle into which the filter is required to fit.

The substrate can be a ceramic, e.g. silicon carbide, cordierite, aluminium nitride, silicon nitride, aluminium titanate, alumina, mullite eg., acicular mullite (see eg. WO 01/16050), pollucite, a thermet such as Al₂O₃/Fe, or composites comprising segments of any two or more thereof. In embodiments wherein the catalyst article of the present comprises a ceramic substrate, the ceramic substrate may be made of any suitable refractory material, e.g., alumina, silica, titania, ceria, zirconia, magnesia, zeolites, silicon nitride, silicon carbide, zirconium silicates, magnesium silicates, aluminosilicates and metallo aluminosilicates (such as cordierite and spodumene), or a mixture or mixed oxide of any two or more thereof. Cordierite, a magnesium aluminosilicate, and silicon carbide are particularly preferred.

It will be understood that a benefit of filters for use in the invention is substantially independent of the porosity of the substrate. Porosity is a measure of the percentage of void space in a porous substrate and is related to backpressure in an exhaust system: generally, the lower the porosity, the higher the backpressure. However, the porosity of filters for use in the present invention are typically >40% or >50% and porosities of 45-75% such as 50-65% or 55-60% can advantageously be used.

The mean pore size of the washcoated porous substrate is important for filtration. So, it is possible to have a porous substrate of relatively high porosity that is a poor filter because the mean pore size is also relatively high. The mean pore size of surface pores of the porous structure of the porous filter substrate can be from 8 to 45 μm, for example 8 to 25 μm, 10 to 20 μm or 10 to 15 μm. The characteristics of substrates, such as pore size, are well known in the art and appropriate measurement techniques are known to the person skilled in the art.

The plurality of inlet channels comprise a first catalyst composition extending from the inlet or the outlet end for at least 50% of L and the plurality of outlet channels comprise a second catalyst composition extending from the outlet or the inlet end for at least 50% of L. The first and second catalyst compositions overlap by at most 80% of L. Preferably the first and second catalyst compositions overlap by at most 20% of L and more preferably by at most 10% of L. For an overlap of at most 80%, one of the first and second compositions may provide a complete coating of the plurality of inlet or outlet channels.

When the overlap is at most 20%, the first catalyst composition may be provided on the inlet channel extending from the inlet end by a distance of at least 50% (of L) and at most 70%, while the second catalyst composition may be provided on the outlet channel extending from the outlet end by a distance of at least 50% and at most 70%. Since the overlap is at most 20%, when the first catalyst composition is, for example 70%, the second catalyst composition can only extend to 50%.

Alternatively, when the overlap is at most 20%, the first catalyst composition may be provided on the inlet channel extending from the outlet end by a distance of at least 50% (of L) and at most 70%, while the second catalyst composition may be provided on the outlet channel extending from the inlet end by a distance of at least 50% and at most 70%. Since the overlap is at most 20%, when the first catalyst composition is, for example 70%, the second catalyst composition can only extend to 50%.

The location of the first and second compositions on the substrate will depend on the coating technique used. At least a portion of the coating is provided on-wall in the region described above. For example, it is straight-forward to coat the inlet channels from the inlet end by simply dipping the substrate in a washcoat. The washcoat may at least partially infiltrate the pores of the substrate during the coating, depending on the particle size and viscosity of the washcoat. Thus, even if a portion of the coating by this technique is in-wall, a portion remains on-wall.

When coating with techniques such as Johnson Matthey's precision coating process described in WO 99/47260, which relies on vacuum or pressure drawing of the washcoat into the substrate, it is possible to apply coatings on the inlet end of the outlet channels and the outlet end of the inlet channels. This technique provides washcoat within the pores of the substrate, but also draws material through to provide an on-wall coating on the opposite side of the substrate from where it was applied.

The inventors have also discovered that the zoned application of the TWC composition onto the wall-flow filter as described herein can further improve the particulate filtration. In particular, when coating with a vacuum infiltration method, such as disclosed in WO 99/47260 using conditions that “pull” the particles in the washcoat from a side of the wall in the wall-flow filter construction onto which the washcoat composition is first contacted through the porous filter wall so that some of them rest on-wall on the opposite (second) side of the wall.

Very surprisingly, a washcoat comprising a combination of sub-micron particles and micron-scale particles arranged on a filter, wherein the outlet channels of the wall-flow filter have an on-wall wedge-shaped profile, wherein a thickest end of the wedge is at the plug end of the channel and washcoat is also present in-wall can filter gasoline particles in the sub-23 micrometer range more effectively than prior art filters.

This arrangement is different from Applicant's WO 2017/056067. WO2017/056067 discloses a catalytic wall-flow filter comprising a three-way catalyst washcoat arranged on-wall in a “wedge” shape when the wall-flow filter is viewed in longitudinal section. That is, the thickness of the on-wall coating tapers from a thickest end at an open channel end of the wall-flow filter

The first and second catalyst compositions are preferably both TWC compositions. TWC compositions are well known in the art and the specific components may readily be selected by a person skilled in the art. The TWC typically comprises one or more platinum group metals (PGM) provided on a high surface area support (i.e. the inorganic oxide), together with an oxygen storage component (OSC) which typically comprises ceria. TWC compositions are generally provided in washcoats onto the substrate.

Each of the first and second catalyst compositions independently comprises a particulate oxygen storage component (OSC) and a particulate inorganic oxide. By particulate it is meant that the components have a particle size distribution. Particle size distributions can be characterized by the D10, D50 and D90 measurements. In each case the number indicates the percentage amount of particles smaller than the recited value. In other words, a D90 of 100 microns means that 90% of the particles are smaller than 100 microns in diameter. By knowing a D10 and a D90, the range of particles in a distribution of particles can be sharply defined.

The particle size measurements necessary to obtain D10, D50 and D90 values are obtained by Laser Diffraction Particle Size Analysis using a Malvern Mastersizer 2000, which is a volume-based technique (i.e. D50 and D90 may also be referred to as D_(v)50 and D_(v)90 (or D(v,0.50) and D(v,0.90)) and applies a mathematical Mie theory model to determine a particle size distribution. The laser diffraction system works by determining diameters for the particles based on a spherical approximation. Diluted washcoat samples were prepared by sonication in distilled water without surfactant for 30 seconds at 35 watts.

The first and second catalyst compositions each independently comprise a particulate oxygen storage component (OSC) having a first D90 and a particulate inorganic oxide having a second D90 and:

-   -   i) the first D90 is less than 1 micron and the second D90 is         from 1 to 20 microns; or     -   ii) the second D90 is less than 1 micron and the first D90 is         from 1 to 20 microns.

That is, of the two oxide components typically present in a TWC composition, one is provided which has a much smaller than conventional particle size. This finer size seems to unexpectedly improve the filtration properties relative to very fine particulate matter.

Preferably i) the first D90 is less than 1 micron and the second D90 is from 5 to 20 microns, or ii) the second D90 is less than 1 micron and the first D90 is from 5 to 20 microns. More preferably the larger particle size distribution is from 5 to 10 microns.

Preferably the particulate OSC has a first D10 and the particulate inorganic oxide has a second D10, wherein:

-   -   i) when the first D90 is less than 1 micron and the second D90         is from 1 to 20 microns, the first D10 is at least 100 nm and         the second D10 is at least 500 nm (preferably at least 1         micron); or     -   ii) when the second D90 is less than 1 micron and the first D90         is from 1 to 20 microns, the second D10 is at least 100 nm and         the first D10 is at least 500 nm (preferably at least 1 micron).

The definition of the particle sizes by their D10 and D90 values clearly defines the breadth of the particle-size distribution. The closer these values are, the narrower the distribution of sizes.

An OSC is an entity that has multi-valence state and can actively react with oxidants such as oxygen or nitrous oxides under oxidative conditions, or reacts with reductants such as carbon monoxide (CO) or hydrogen under reducing conditions. Examples of suitable oxygen storage components include ceria. Praseodymia can also be included as an OSC. Delivery of an OSC to the washcoat layer can be achieved by the use of, for example, mixed oxides. For example, ceria can be delivered by a mixed oxide of cerium and zirconium, and/or a mixed oxide of cerium, zirconium, and neodymium. Preferably, the OSC comprises or consists of one or more mixed oxides. The OSC can be ceria or a mixed oxide comprising ceria. The OSC may comprise a ceria and zirconia mixed oxide; a mixed oxide of cerium, zirconium, and neodymium; a mixed oxide of praseodymium and zirconium; a mixed oxide of cerium, zirconium and praseodymium; or a mixed oxide of praseodymium, cerium, lanthanum, yttrium, zirconium and neodymium. Preferably the OSC of the first and second catalyst compositions are each independently selected from the group consisting of cerium oxide, a ceria-zirconia mixed oxide, and an alumina-ceria-zirconia mixed oxide. The ceria-zirconia mixed oxide can have a molar ratio of zirconia to ceria at least 50:50, preferably, higher than 60:40.

Preferably the particulate inorganic oxide is selected from the group consisting of alumina, magnesia, silica, lanthanum, neodymium, praseodymium, yttrium oxides, and mixed oxides or composite oxides thereof. Preferably the particulate inorganic oxide, which may be provided as a support for PGMs, is independently selected from the group consisting of alumina, silica-alumina, alumino-silicates, alumina-zirconia, and alumina-ceria. Preferably the particulate inorganic oxide has a surface area of at least 80 m²/g, more preferably at least 150 m²/g and most preferably at least 200 m²/g.

Preferably the first and/or second catalyst composition further comprises a first platinum group metal (PGM) component, preferably selected from the group consisting of platinum, palladium, rhodium, and a mixture thereof. The first and/or second catalyst composition may comprise 1-100 g/ft³ of the PGM component, preferably, 5-80 g/ft³, more preferably, 10-50 g/ft³ of the PGM component.

Preferably the first and/or second catalyst composition further comprises a first alkali or alkali earth metal component, preferably wherein the first alkali or alkali earth metal is barium or strontium. Preferably the barium or strontium, where present, is present in an amount of 0.1 to 15 weight percent, and more preferably 3 to 10 weight percent barium, based on the total weight of the first and or second catalyst. Preferably the barium is present as a BaCO₃ composite material. Such a material can be performed by any method known in the art, for example incipient wetness impregnation or spray-drying. Alternatively, barium hydroxide can be used in the catalyst article.

The catalyst article of the invention may comprise further components that are known to the skilled person. For example, the compositions of the invention may further comprise at least one binder and/or at least one surfactant. Where a binder is present, dispersible alumina binders are preferred.

Preferably the first and second catalyst compositions are substantially the same. Although the compositions may be the same, the loading may differ. Preferably, the first and second catalyst compositions may be the same, in order to reduce manufacturing complexity.

Preferably the first and/or second catalyst composition have a ratio by weight of the particulate OSC to the particulate inorganic oxide of from 10:1 to 1:10, preferably from 3:1 to 1:3. Preferably the loading of the OSC in each composition is from 0.5 to 4 g/in³, preferably from 1 to 3 g/in³.

Preferably the first and/or second catalyst compositions are provided directly on the substrate, such as by washcoating as described above.

According to a further aspect there is provided an exhaust gas treatment system comprising the catalyst article as described herein in fluid communication with an exhaust manifold of a positive ignition engine.

As desired, the exhaust system can also include additional components. For example, in exhaust systems applicable particularly to lean-burn engines, a NOx trap can be disposed either upstream of the catalyst article described. A NOx trap, also known as a NOx absorber catalysts (NACs), are known e.g. from U.S. Pat. No. 5,473,887 and are designed to adsorb nitrogen oxides (NOx) from lean (oxygen rich) exhaust gas (lambda >1) during lean running mode operation and to desorb the NOx when the oxygen concentration in the exhaust gas is decreased (stoichiometric or rich running modes). Desorbed NOx may be reduced to N₂ with a suitable reductant, e.g. gasoline fuel, promoted by a catalyst component, such as rhodium or ceria, of the NAC itself or located downstream of the NAC.

As desired, after leaving the catalyst article described herein, optionally the exhaust gas stream can next be conveyed via an appropriate exhaust pipe to a downstream NOx trap for adsorbing any remaining NOx emission contaminants in the exhaust gas stream. From the NOx trap through a further exhaust pipe, a SCR catalyst can be disposed to receive the outlet of the NOx trap to provide further emissions treatment of any ammonia generated by the NOx trap with a selective catalytic reduction catalyst for reducing oxides of nitrogen to form nitrogen and water using the ammonia as reductant. From the SCR catalyst, an exhaust pipe can lead to the tail pipe and out of the system.

According to a further aspect there is provided a method of treating exhaust gas emitted from a positive ignition internal combustion engine, the method comprising contacting the exhaust gas with the catalyst article as described herein. Preferably the engine is a lean-burn gasoline engine. Additionally, the present disclosure can comprise a vehicle, such as a passenger vehicle, comprising an engine as described herein.

According to a further aspect there is provided the use of the catalyst article as described herein in an exhaust gas treatment system for reducing the emission of particles smaller than 23 nm, in particular for reducing the emission of particles from 10 to 23 nm. Preferably the use reduces the emission of particles smaller than 23 nm by at least 20 percent by number; more preferably, at least 30 percent by number; and even more preferably, at least 40 percent by number.

According to a further aspect there is provided a method of treating exhaust gas emitted from a positive ignition internal combustion engine, the method comprising contacting the exhaust gas with the catalyst article as described herein, wherein the method is for reducing the emission of particles smaller than 23 nm, in particular for reducing the emission of particles from 10 to 23 nm.

According to a further aspect there is provided the use of a catalyst article in an exhaust gas treatment system for reducing the emission of particles smaller than 23 nm, in particular for reducing the emission of particles from 10 to 23 nm, wherein the catalyst article is for treating an exhaust gas from a positive-ignition internal-combustion engine, the article comprising:

a substrate which is a wall-flow filter having an inlet end and an outlet end and an axial length L therebetween, a plurality of inlet channels extending from the inlet end and a plurality of outlet channels extending from the outlet end,

wherein the plurality of inlet channels comprise a first catalyst composition extending from the inlet or outlet end for at least 50% of L and the plurality of outlet channels comprise a second catalyst composition extending from the outlet or inlet end for at least 50% of L, and

wherein the first and second catalyst compositions each independently comprise a particulate oxygen storage component (OSC) having a first D90 and a particulate inorganic oxide having a second D90 and:

-   -   i) the first D90 is less than 1 micron and the second D90 is         from 1 to 20 microns; or     -   ii) the second D90 is less than 1 micron and the first D90 is         from 1 to 20 microns.

The term “washcoat” is well known in the art and refers to an adherent coating that is applied to a substrate usually during production of a catalyst.

The acronym “PGM” as used herein refers to “platinum group metal”. The term “platinum group metal” generally refers to a metal selected from the group consisting of Ru, Rh, Pd, Os, Jr and Pt, preferably a metal selected from the group consisting of Ru, Rh, Pd, Jr and Pt. In general, the term “PGM” preferably refers to a metal selected from the group consisting of Rh, Pt and Pd.

The term “mixed oxide” as used herein generally refers to a mixture of oxides in a single phase, as is conventionally known in the art. The term “composite oxide” as used herein generally refers to a composition of oxides having more than one phase, as is conventionally known in the art.

The expression “consist essentially” as used herein limits the scope of a feature to include the specified materials, and any other materials or steps that do not materially affect the basic characteristics of that feature, such as for example minor impurities. The expression “consist essentially of” embraces the expression “consisting of”.

The expression “substantially free of” as used herein with reference to a material, typically in the context of the content of a region, a layer or a zone, means that the material in a minor amount, such as ≤5% by weight, preferably ≤2% by weight, more preferably ≤1% by weight. The expression “substantially free of” embraces the expression “does not comprise.”

The expression “essentially free of” as used herein with reference to a material, typically in the context of the content of a region, a layer or a zone, means that the material in a trace amount, such as ≤1% by weight, preferably ≤0.5% by weight, more preferably ≤0.1% by weight. The expression “essentially free of” embraces the expression “does not comprise.”

Any reference to an amount of dopant, particularly a total amount, expressed as a % by weight as used herein refers to the weight of the support material or the refractory metal oxide thereof.

The term “loading” as used herein refers to a measurement in units of g/ft³ on a metal weight basis, unless other units are provided.

The invention will now be described in relation to the following non-limiting figures, in which:

FIG. 1 shows a schematic of a wall-flow filter as described herein.

FIG. 2 shows a schematic of a wall-flow filter as described herein.

FIG. 3 shows particulate emissions and specifically those between 10-23 nm for exemplary catalyst articles, in particular, simulated WLTC engine bench data for 10-23 nm Particulate count in #/km.

FIG. 1 shows a cross-section through a wall-flow filter 1 made of a porous cordierite substrate. The wall-flow filter 1 has a length L extending between the inlet end 5 and the outlet end 10. The wall-flow filter 1 comprises a plurality of inlet channels 15 and a plurality of outlet channels 20 (only one shown). The plurality of inlet channels 15 and outlet channels 20 are plugged at their respective ends with plugs 21.

The internal surfaces 25 of the inlet channels 15 have been coated with an on-wall coating 30 comprising a first catalyst composition. The coating 30 extends at least 50% of L from the inlet end 10.

The internal surfaces 35 of the outlet channels 20 have been coated with an on-wall coating 40 comprising a second catalyst composition. The coating 40 extends at least 50% of L.

The overlap region 45 between coatings 30 and 40 is less than or equal to 20% of L.

In use, an exhaust gas flow 50 (shown with the arrows) passes into the inlet channels 15, through the walls of the wall-flow filter 1, and out of the outlet channels 20.

FIG. 2 shows a cross-section through a wall-flow filter 1 made of a porous cordierite substrate. The wall-flow filter 1 has a length L extending between the inlet end 5 and the outlet end 10. The wall-flow filter 1 comprises a plurality of inlet channels 15 and a plurality of outlet channels 20 (only one shown). The plurality of inlet channels 15 and outlet channels 20 are plugged at their respective ends with plugs 21.

The internal surfaces 25 of the inlet channels 15 have been coated with an on-wall coating 30 comprising a first catalyst composition. The coating 30 extends at least 50% of L from the outlet end 10. The coating 30 is wedge-shaped with a thicker amount at the outlet end 10. The coating 30 is applied in accordance with the method of WO 99/47260.

The internal surfaces 35 of the outlet channels 20 have been coated with an on-wall coating 40 comprising a second catalyst composition. The coating 40 extends at least 50% of L from the inlet end 5. The coating 40 is wedge-shaped with a thicker amount at the inlet end 5. The coating 40 is applied in accordance with the method of WO 99/47260.

The overlap region 45 between coatings 30 and 40 is less than or equal to 20% of L.

In use, an exhaust gas flow 50 (shown with the arrows) passes into the inlet channels 15, through the walls of the wall-flow filter 1, and out of the outlet channels 20.

The invention will now be described in relation to the following non-limiting examples.

Materials

All materials are commercially available and were obtained from known suppliers, unless noted otherwise.

EXAMPLE 1

A three way catalyst washcoat was prepared at a washcoat loading of 1.6 g/in³ (1.2 g/in³ CeZr mixed oxide and 0.4 g/in³ alumina) and a PGM loading of 22 g/ft³ (Pt:Pd:Rh, 0:20:2): comprised of a Rare Earth Oxide (REO) Oxygen Storage Component (OSC) (weight ratio of ZrO₂ to CeO₂ is about 2:1) with a D90 of <1 μm and a La-stabilized alumina component which was wet milled to a D90 of 12 μm.

The completed washcoat was adjusted to a suitable final washcoat solids content in order to coat onto the GPF substrate using Johnson Matthey's precision coating process described in WO 99/47260. The substrate used was a commercially available cordierite GPF substrate of a nominal 63% porosity and 17.5 μm mean pore size and of dimensions 4.66 inch diameter by 6 inch in length, 300 cells per square inch and a channel wall thickness of 8 thousandths of an inch. The coating was applied from each end of the substrate with each application covering a length between 50 and 65% of to achieve a fully coated final product with no uncoated region. The coated part was then dried and calcined in the normal way known to the art.

EXAMPLE 2

Example 2 was prepared in the same way as Example 1 but the alumina component was wet milled further to a D90 of 5 μm.

COMPARATIVE EXAMPLE 3

A reference coated GPF was prepared by applying a current state of art TWC coating developed for GPF substrate. The washcoat loading was prepared at 1.6 g/in³ (1.2 g/in³ CeZr mixed oxide and 0.4 g/in³ alumina) and the PGM loading 22 g/ft³ (Pt:Pd:Rh, 0:20:2). In contrast to Examples 1 and 2, here both the OSC component and alumina component have a D90 of 7 μm.

Uncoated 220/6 Reference GPF

For the purposes of further comparison to the invention, an uncoated GPF reference has been used. This is a commercially available GPF substrate which may be considered for use in such aftertreatment systems for Euro 6 PN limit of 6×10¹¹. The same catalyst diameter and length and therefore catalyst volume was selected as Examples 1-3, 4.66″ diameter and 6″ length. However, to achieve the required filtration efficiency, such bare filters require lower nominal porosity and mean pore size. Since this can cause an undesirable increase in backpressure, the cells per square inch and wall thickness are typically lower than the substrates which are designed to be coated with washcoat. The uncoated reference GPF was of 220 cells per square inch and a wall thickness of 6 thousandths of an inch.

Filtration Performance Evaluation

The above examples were evaluated for particulate emissions and specifically those between 10-23 nm. The evaluation was carried out over a simulated WLTC performed on an engine bench equipped with a 2.0 L Gasoline Direct Injection engine. The particles are measured using a Combustion DMS500 particle analyser.

The results are shown in FIG. 3 and were an average of 3 WLTC tests completed for each example. The results clearly show the benefit of Examples 1 and 2 in comparison to the uncoated low porosity 220/6 GPF and Comparative Example 3. Particularly, Example 1 shows a much improved benefit over the currently available alternatives.

Table 1 below demonstrates additional reduced backpressure benefit of Example 1 and Example 2 vs. Comparative Example 3.

TABLE 1 Cold Flow BP at Sample 600 m³/hr (mbar) Bare 220/6 44.1 Comparative 65.7 Example 3 Example 1 63.0 Example 2 56.4 Unless otherwise stated, all percentages herein are by weight.

Although preferred embodiments of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the scope of the invention or of the appended claims. 

The invention claimed is:
 1. A catalyst article for treating an exhaust gas from a positive-ignition internal-combustion engine, the article comprising: a substrate which is a wall-flow filter having an inlet end and an outlet end and an axial length L therebetween, a plurality of inlet channels extending from the inlet end and a plurality of outlet channels extending from the outlet end, wherein the plurality of inlet channels comprise a first catalyst composition extending from the inlet or outlet end for at least 50% of L and the plurality of outlet channels comprise a second catalyst composition extending from the outlet or inlet end for at least 50% of L, wherein the first and second catalyst compositions overlap by at most 80% of L, and wherein the first and second catalyst compositions each independently comprise a particulate oxygen storage component (OSC) having a first D90 and a particulate inorganic oxide having a second D90 and: i) the first D90 is less than 1 micron and the second D90 is from 1 to 20 microns; or ii) the second D90 is less than 1 micron and the first D90 is from 1 to 20 microns.
 2. The catalyst article according to claim 1, wherein the particulate OSC has a first D10and the particulate inorganic oxide has a second D10, wherein: i) when the first D90 is less than 1 micron and the second D90 is from 1 to 20 microns, the first D10 is at least 100 nm and the second D10 is at least 500 nm; or ii) when the second D90 is less than 1 micron and the first D90 is from 1 to 20 microns, the second D10 is at least 100 nm and the first D10 is at least 500 nm.
 3. The catalyst article according to claim 1, wherein: i) the first D90 is less than 1 micron and the second D90 is from 5 to 20 microns, or ii) the second D90 is less than 1 micron and the first D90 is from 5 to 20 microns.
 4. The catalyst article according to any of claim 1, wherein the first and second catalyst compositions overlap by at most 20% of L.
 5. The catalyst article according to claim 1, wherein the particulate OSC is selected from the group consisting of cerium oxide, zirconium oxide, a ceria-zirconia mixed oxide, and an alumina-ceria-zirconia mixed oxide.
 6. The catalyst article of claim 1, wherein particulate inorganic oxide is selected from the group consisting of alumina, magnesia, silica, lanthanum, neodymium, praseodymium, yttrium oxides, and mixed oxides or composite oxides thereof.
 7. The catalyst article of claim 1, wherein the first and/or second catalyst composition further comprises a first platinum group metal (PGM) component selected from the group consisting of platinum, palladium, rhodium, and a mixture thereof.
 8. The catalyst article of claim 1, wherein the first and/or second catalyst composition further comprises barium or strontium.
 9. The catalyst article according to claim 1, wherein the first and second catalyst compositions are the same.
 10. The catalyst article according to claim 1, wherein the first and/or second catalyst composition have a ratio by weight of the particulate OSC to the particulate inorganic oxide of from 3:1 to 1:3.
 11. The catalyst article of claim 1, wherein the first and/or second catalyst compositions are provided directly on the substrate.
 12. An exhaust gas treatment system comprising the catalyst article according to claim 1 in fluid communication with an exhaust manifold of a positive ignition engine.
 13. A method of treating exhaust gas emitted from a positive ignition internal combustion engine, the method comprising contacting the exhaust gas with the catalyst article of claim
 1. 14. Use of the catalyst article according to claim 1 in an exhaust gas treatment system for reducing the emission of particles smaller than 23 nm.
 15. The use according to claim 14, wherein the use reduces the emission of particles smaller than 23 nm by at least 20% by number. 